The terms written in the lease are binding. He cannot change the rent after you've signed the lease, even if he made a mistake, because $775 is the amount you agreed to pay. You cannot be required to pay more. You're not short on the rent as long as you pay the amount written in the lease and he CANNOT evict you for non-payment of rent or pay late charges. A lease is a contract, and it must be followed, especially when you had no possible way of knowing he made a mistake.

The only thing you need to do is keep paying your normal, agreed rent amount. You can, however, explain that you are not obligated to pay more due to his unilateral mistake. The contract is valid as written.

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